The present invention relates to a solenoid-operated valve and more particularly, but not exclusively, to such a valve for use underwater in a control system of a subsea wellhead for the extraction of oil and/or gas.
In subsea drilling, large actuators or process valves associated with the wellhead are typically controlled by smaller main-stage valves and pilot-operated valves. These smaller valves are hydraulically and/or electrically-operated and may be housed within a subsea container known as a control “pod” so that they are protected from the surrounding environment. The pod is sealed against the ingress of sea water and has a one or more connectors for connection to a supply of hydraulic fluid and/or electric power for operation of the valves. The pod is typically filled with a suitable oil so that it immerses and enters the valves. The oil typically contains a corrosion inhibitor and serves as an electrical insulator.
Some valves may be designed such that they are sealed against the ingress of pod oil and are instead flooded with hydraulic control fluid.
In some instances it is possible for sea water to ingress into the pod. For example, the pod may develop a leak. In the instance where the valves are flooded with pod oil, the ingress of seawater into the pod can lead to seawater entering the valves which undesirable as it increases the risk of corrosion of valve parts. In the instance where valves are flooded with hydraulic control fluid there is still a risk of seawater entering the valve via connectors disposed on the pod for connection to corresponding connectors provided on the end of an umbilical (a collection of lines, hoses or tubes through which control fluid and electrical power may be supplied) that extends from a topside production platform or vessel.
The armature of a solenoid-operated valve is designed to have relatively high magnetic permeability so as to ensure that the force applied the actuator is sufficient for to move the valve member without unduly high electrical power being consumed. Such materials are vulnerable to corrosion and it is thus common to apply a suitable coating to the armature such as, for example, nickel plating.
It has been found that the oil or hydraulic control fluid that enters the solenoid cavity from the pod or the rest of the valve can cause the solenoid actuator to jam. In particular, over time the running gap between the solenoid actuator and the surrounding armature becomes filled with a sticky deposit or gum-like substance. It is thought that the problem may be caused by the corrosion inhibitor and/or other additives in the oil/control fluid chemically degrading the nickel plating on the armature.
The content of hydraulic control fluid is regulated by industry or government standards to avoid harmful substances being vented into the sea. Such control fluids have a high water content and the volume of additives is being increasingly restricted for environmental reasons. This increases the risk of corrosion of valve parts and, in particular, solenoid operators.
It is an object of the present invention to obviate or mitigate the aforementioned disadvantages. It is also an object to provide for an alternative or improved solenoid-operated valve.